Graphene, as one of various carbon structures such as graphite, carbon nanotubes, and fullerene, is a two-dimensional structure that is composed of honeycomb structures in which carbon atoms are connected like a net.
Since graphene not only has high strength and high thermal conductivity but also has very good electrical conductivity and good elasticity, graphene may maintain electrical conductivity even in the case in which it is stretched or folded. Thus, graphene is being used in various application areas.
As one of typical application areas, graphene has a possibility of being used as an ultra high-speed transistor device through structural transformation to nanoribbon graphene, bilayer graphene, or nanomesh graphene due to its field-effect characteristics, and a study on a high-performance flexible transistor has recently been reported [Y-M Lin and Phaedon Avouris, “Strong Suppression of Electrical Noise in Bilayer Graphene Nanodevice,” Nano Lett., February 2008, pp. 2119-2125; Jingwei Bai, Xing Zhong, Shan Jiang, Yu Huang, and Xiangfeng Duan, “Graphene Nanomesh,” Nature Nanotech., February 2010, pp. 190-194; B. J. Kim, H. Jang, S. K. Lee, B. H. Hong, J. H. Ahn, and J. H. Cho, “High-Performance Flexible Graphene Field Effect Transistors with Ion Gel Gate Dielectrics?,” Nano Lett., 2010, pp. 3464-3466].
Also, electrical, mechanical, and thermal properties of graphene are applied to a biomimetic device to result in excellent characteristics in which the device has large displacement and fast response rate even at low power and the displacement also increases as the temperature increases [Shou-En Zhu et al., “Graphene-based Bimorph Microactuators,” Nano Lett., January 2011, pp. 977-981].
Korean Patent Application Laid-Open Publication No. 2011-0105408 discloses that a flexible resistance variable memory device using a graphene oxide is manufactured by the steps of forming a lower electrode layer on a substrate, forming a graphene oxide layer on the formed lower electrode, and forming an upper electrode layer on the formed graphene oxide layer.
In addition, Thomas Mueller et. al. have estimated that an ultra high-speed photodetector, which may operate at a wider wavelength range and may have a very fast response rate, may be realized using the properties of graphene [Thomas Mueller, Fengnian Xia, and Phaedon Avouris, “Graphene Photodetectors for High-Speed Optical Communications,” Nature Photonics, March 2010, pp. 297-301].
Furthermore, results of measuring characteristics of graphene that is grown by chemical vapor deposition (CVD) and transferred on the surface of a metamaterial have recently been reported [Nikitas Papasimakis, Zhiqiang Luo, Ze Xiang Shen, Francesco De Angelis, Enzo Di Fabrizio, Andrey E. Nikolaenko, and Nikolay I. Zheludev, “Graphene in a Photonic Metamaterial,” Optics Express, April 2010, pp. 8353-8359].
As described above, the application areas of graphene are very diverse according to various properties, and its potential applications are also very wide.
Thus, research into replacing transparent conductive oxides (TCOs), such as indium tin oxide (ITO) and zinc tin oxide (ZTO), which have been used as a transparent electrode, with graphene by using its advantages such as high transparency, high conductivity, high flexibility, and low manufacturing cost, has been continuously conducted.
Graphene has been prepared using chemical synthesis, CVD, and epitaxial synthesis, after it was first discovered through mechanical exfoliation (the so-called “Scotch tape method”) by researchers in 2004.
Currently, highly crystalline graphene is prepared using mechanical cleavage of graphite, but it may have a low yield. Also, large-area monolayer graphene may be prepared by a method using the sublimation of SiC. However, it may have a low yield and coating (transferring) on another substrate may be difficult. Furthermore, when CVD is used, a large-area substrate may be coated with good-quality graphene, and thus, its applicability to a transparent conductive film (TCF) has increased. However, there is a need to overcome a limited area of the substrate.
Specifically, Japanese Patent Application Laid-Open Publication No. 22-153793 discloses a technique of horizontally stacking graphene by a CVD method, Korean Patent Application Laid-Open Publication No. 2011-0081519 discloses a graphene nanostructure prepared using a self-assembling material, and Korean Patent Application Laid-Open Publication No. 2010-121978 discloses a method of forming a graphene thin film by spray deposition of a graphene dispersion.
The graphene prepared by the above methods is stacked in a two-dimensional structure and does not have electrical conductivity in a vertical direction.
The graphene proposed in the above patents has a vertically stacked structure. However, since crystal planes of the graphene are horizontally deposited when the arrangement of the graphene is observed, it may be understood that the graphene itself is maintained in a two-dimensional structure.
According to electrical conduction properties of graphene, since electrons tend to move along the edges of the graphene, effects unexpected from a two-dimensional structure may be obtained when the graphene is formed in a three-dimensional structure instead of a two-dimensional structure.
As described above, a typical technique of applying graphene to a transparent electrode is widely known. However, a method of preparing a large-area graphene transparent electrode is still in a beginning stage. In Korean Patent Application Laid-Open Publication No. 2010-121978, a graphene thin film is prepared by spray deposition, but satisfiable, uniform thin film properties may be difficult to be obtained when the spay deposition is applied to prepare a large-area graphene thin film.
In relation to a technique of preparing a large-area thin film, there is an electrospray process in which a uniform thin film may be obtained by the deposition of droplets on a substrate. For example, Korean Patent Application Laid-Open Publication No. 2010-42345 discloses a method of fabricating an organic thin film, i.e., a patterned mask, through an electrospray process. However, it is difficult to find an example of applying an electrospray process to graphene so far.